Conventionally, ion implantation has been widely used as a method of doping impurities used in manufacturing a semiconductor device. As a recent promising technology, attention is now focused on plasma-doping suitable for forming a shallow junction. Plasma-doping is a technique in which a surface of an object to be processed, for example, a semiconductor substrate, undergoes irradiation of plasma containing impurities, i.e., dopant so that dopant is doped therein.
In ion implantation, through a mass segregation process, an ion to be doped is extracted from plasma containing dopant and accelerated before being doped into an object to be processed. On the other hand, in plasma-doping, an object to be processed undergoes direct irradiation of plasma containing dopant so that the dopant is doped into relatively shallow surface regions. In this case, the plasma contains compound gases having dopant, ions and radicals of the compound, and ions and radicals of isolated dopant. When neutral gas having no dopant is added to plasma, the plasma further contains ions and radicals of the neutral gas.
In ion implantation, an intended dopant is ionized in advance, and therefore the dopant is directly measured with a Faraday cup as the amount of the ion. An amount of dopant to be added has been controlled according to the ion amount. In plasma-doping, on the other hand, it is thought that not only ions of a compound containing dopant but also the radicals are doped as dopant into the surface of an object to be processed. Radical, which is often translated into free radical, means an atom or molecule having at least one unpaired electron. Generally, such a state is very active and therefore easily causes reaction. Plasma contains gases including compounds of dopant in various forms: ions, radicals, and neutral gas. Therefore, in plasma-doping, the amount of dopant depends on doping conditions under which the dopant is doped from plasma into an object to be processed. To control the amount of dopant, many methods have been developed. As a typically used controlling method, ions that collide with the object to be processed are measured during plasma-doping as an amount of electric charge and by which the amount of dopant is controlled. As a measurement tool, a Faraday cup, which is capable of measuring the quantity of ions that collide with the object, is often used. For example, U.S. Pat. No. 6,020,592 and Reference 1 (Proc. 2000 Int. Conf. on Ion Implant. Tech., Alpbach, Austria, 17-22 Sep. 2000) disclose a method of measuring an amount of impurities, i.e., dopant while plasma-doping is being carried out. The measurement in the method employs an advanced Faraday cup. According to the method, a dose amount is determined on measurement of the amount of electric charge of ions that moved from plasma to a semiconductor wafer as the object to be processed. Specifically, a Faraday cup is disposed adjacent to the semiconductor wafer to measure the amount of ions. Real-time feedback of a current value measured by the Faraday cup controls the amount of dopant to be doped into the semiconductor wafer.
When the plasma doping is used in a manufacturing process of a semiconductor device, measurement of the amount of ions by a Faraday cup during the plasma doping controls the dose amount, and at the same time, the crystalline state of the object is monitored by in-situ observation with the use of surface optical absorption measurement or an ellipsometer.
The conventional dose-amount control above, however, has a pending problem in the plasma-doping process on a semiconductor device where precise control of the dose amount is required. Due to variations or errors in the dose amount, variations in semiconductor characteristics cannot be reduced. That is, the conventional methods have offered an insufficient consistency in correlation between the dose amount estimated from measurement of ions included in plasma and the dose amount in a semiconductor wafer after doping measured by secondary ion mass spectrometry (SIMS) or the like. Furthermore, SIMS is a destructive measurement where a part of a wafer has to be broken down, and therefore a sampling process is required in mass production.
The object of the present invention is to provide an impurity doping control method capable of controlling a dose amount of dopant with high accuracy and to provide an impurity doping apparatus.